High Irradiance Laser Ionization MassSpectrometry for Direct Speciation of Iron Oxides

Bin Yan,a Lingfeng Li,a Quan Yu,a Wei Hang,a,b Jian He,c andBenli Huanga

a Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University,Xiamen, Chinab State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, Chinac Department of Mechanical and Electrical Engineering, Xiamen University, Xiamen, China

A novel method has been developed that allows the direct speciation analysis of iron oxidesbased on a modified laser ionization orthogonal time-of-flight mass spectrometer. Timeresolved mass spectra were acquired for the investigation of elemental ions and oxide ionsgenerated by a laser ionization source. Speciation methodologies, including the identificationof characteristic ions and the use of ion abundance ratios were evaluated for the differentiationof the oxides. The influence of operating parameters on the distribution of cluster ions wasinvestigated, and their mechanism of formation discussed. (J Am Soc Mass Spectrom 2010,21, 1227–1234) © 2010 American Society for Mass Spectrometry

Traditionally, mineral analysis focuses on thequantitative determination of the principle com-position and impurity elements. Usually, samples

are dissolved and analyzed by related instrumentationmethods, such as atomic emission, absorption, fluores-cence spectroscopy (AES, AAS, AFS) [1], and plasmamass spectrometry (MS) [2] for quantitative analysis.Conventional solution-based analytical techniques suf-fer from trivial manipulation, long processing time, andthe presence of foreign contaminants. As a result,methods involving direct analysis of solids attract themost attention. In addition to well established X-rayfluorescence (XRF) [3], several MS based methods forthe analysis of solids have also been widely used ininorganic analysis, including laser ablation inductivelycoupled plasma mass spectrometry (LA-ICPMS) [4],glow discharge mass spectrometry (GDMS) [5], second-ary ion mass spectrometry (SIMS) [6–9], and laserionization mass spectrometry (LIMS) [10 –16]. Never-theless, the quantification of the elements present isnot enough for sample evaluations because the oxi-dation state of a particular element may have greatinfluence on the related toxicity, catalytic perfor-mance, or refining method. Hence, speciation analy-sis is often required.

For speciation analysis, both XRF and MS couldprovide useful information. Using the XRF method,direct speciation could be carried out through investi-gations of energy shifts or intensity ratios of fluores-cence lines [17]. However, these methods are not able todistinguish oxides from salts because the detection of

Address reprint requests to Professor W. Hang, Department of Chemistry,

College of Chemistry and Chemical Engineering, Xiamen University, No.422, Siming South Road, Xiamen, China. E-mail: weihang@xmu.edu.cn

© 2010 American Society for Mass Spectrometry. Published by Elsevie1044-0305/10/$32.00doi:10.1016/j.jasms.2010.02.030

line energy, width, or intensity ratios lead to nearlyidentical results. In contrast, an MS method would notsuffer from these problems, as the mass spectrum ofbinary salts, oxides, and oxysalts could easily be iden-tified. Nevertheless, several obvious faults exist forsome MS ionization methods. Conventional GDMS usessample conductivity and requires analytical gradegraphite or metal powder mixtures of insulative oxides[5], which increases the time and cost of sample prep-aration. Although there are no special requirements forsample properties in SIMS analysis, the limited sput-tered ions, pollutants, or the superficial oxide layer,which is stoichiometrically different from the bulk,often make speciation analysis complicated. Thus, be-fore the acquisition of SIMS data, a pre-sputtering stepmay be necessary.

Due to its versatility, laser ablation is considered apowerful technique in mineral analysis. Currently, thetechnique is prevalently associated with LA-ICPMS,which has been applied to the elemental and isotope-specific analysis of different types of solid materials[18–20] because of its great experimental flexibility.However, cluster ions, which play key roles for specia-tion analysis, would not survive in the ICP plasma.

Up to now, several excellent studies have beenconducted on oxide differentiation by LIMS. Aubriet etal. evaluated the capabilities of laser ionization Fouriertransform mass spectrometry for the speciation of chro-mium compounds and matrix effects [14]. He alsoinvestigated most of the first-row transition-metal ox-ides and found that a strong correlation between theoxides and the cluster ions exists in both positive andnegative ions [21]. Van Vaeck et al. studied the potentialand limitations of laser microprobe mass spectrometry

(LMMS) for inorganic speciation at a microscopic level

Published online March 31, 2010r Inc. Received January 7, 2010

Revised February 21, 2010Accepted February 22, 2010

1228 YAN ET AL. J Am Soc Mass Spectrom 2010, 21, 1227–1234

[22]. By deductive reasoning, compounds from differ-ent classes may be identified. Reference spectra wereneeded for the comparison of compounds with thesame elements in different stoichiometries. Hachimi etal. performed speciation of chromium, nickel, and leadcompounds in solids with LMMS and applied thetechnique to environment and biological tissue analysis[16]. Allen et al. [23] described speciation of arsenicoxides using high-mass cluster ions that are unique tothe oxidation state of each oxide sample in the negativeion mass spectrum. Maunit et al. distinguished variousiron oxides by analyzing the distribution of clusters andexplained cluster formation mechanism using LMMS[24, 25]. De Ville d’Avray et al. investigated ferritenanoparticles by laser desorption ionization mass spec-trometry and found that more high m/z peaks wereproduced in Fe3O4 particles compared with Fe2O3 [26]However, the above studies were conducted with laserirradiation less than 108 W/cm2, which indicates thatthe laser-target interaction mechanism probably in-volves a desorption-ionization process accompanied bythe emission of isolated neutrals, ions, and electrons.For example, ablation-desorption threshold is 4 � 108

and 1 � 108 W/cm2 for V2O3 and V2O4, respectively[10], and about 5 � 108 W/cm2 for CrO3, MoO3, andWO3 at the wavelength of 355 nm [27].

Our home-built laser ionization orthogonal time-of-flight mass spectrometer (LI-O-TOFMS) has been usedin extensive applications, including the direct semi-quantitative multi-elemental analysis of solid-state sam-ples such as alloys, ores, biological materials, andresidues [28–31]. In this study, we demonstrate theutility of the LI-O-TOFMS for the direct speciationanalysis of iron oxides with laser irradiation above 1010

W/cm2, leading to a potentially different ionizationmechanism compared with the previous studies.

Experimental

Instrument

All experiments were performed with an in-house,home-built, laser ionization orthogonal time-of-flightmass spectrometer that had been described previously[29, 31]. However, several modifications have beenmade in the source, transmission, and time-of-flightregion. A Q-switched Nd:YAG laser (5 ns pulse width,Dawa100, Beamtech Inc., Beijing, China) was used inthe experiment. The operating wavelengths were 266,532, and 1064 nm. With the use of a single quartz lens,the laser beam was focused onto the sample surfacewith a focal spot diameter of 50 �m. To reduce massdiscrimination during the transmission, a set of electri-cal static lenses were employed to replace the hexapolesystem used previously [31]. Steering plates wereadded in the TOF region to adjust the trajectory of theions and increase the sensitivity of the instrument. Adigital storage oscilloscope (42Xs, Lecroy, NY, USA)

was utilized for spectrum acquisition. Figure 1 and

Table 1 show the schematic diagram of the newlymodified system and the operating conditions.

In our previous research related to elemental analy-sis, buffer gas was introduced to the ion source undermild pressure for ion cooling and charge reduction [32].However, to obtain information concerning elementalspecies, little buffer gas should be introduced into theionization chamber because frequent collisions couldlead to spectral complication. Thus, the pressure of theionization chamber was maintained at the value nomore than 200 Pa throughout the experiment.

Sample Preparation

The iron oxides investigated in this study includedFe2O3 and Fe3O4 (analytical grade, Sinopharm ChemicalReagent Co., Ltd., Shanghai, China). Before beingmounted onto the direct insertion probe (DIP), thesamples were ground carefully in an agate mortar for 10min to minimize any effect related to particle size. Next,the samples were pressed in a die under a pressure of 5 �107 Pa for 3 min to produce a disk that was 1.5 mm inthickness and 8 mm in diameter. During the experi-ment, the DIP was mechanically rotated continuously toprovide fresh sample surface.

Results and Discussion

Evaluation of Laser Wavelength

Iron oxide speciation analysis was performed previ-ously using LMMS with a laser desorption ionizationmechanism [24, 25]. The conclusion was that the neutralspecies FeO plays an important role in the formation ofvarious cluster ions. At a wavelength of 586 nm (h� �2.11 eV) with laser irradiance of 108 W/cm2, FeO (bondenergy 4.23 eV) can exist, and many large cluster ionsappear in the spectrum. When 279 nm (4.44 eV) and 266nm (4.66 eV) wavelengths are used, the energy of thephoton exceeds the bond energy of FeO, and the large

Figure 1. Schematic diagram of the laser ionization orthogonaltime-of-flight mass spectrometer.

cluster ions disappear. However, in our work, ion

1229J Am Soc Mass Spectrom 2010, 21, 1227–1234 DIRECT SPECIATION BY HIGH IRRADIANCE LIMS

abundance distributions were similar at the three wave-lengths of 1064 nm, 532 nm, and 266 nm. Thus, adifferent mechanism of ion formation may exist be-tween our work and previous research.

Figure 2 shows the LI-TOFMS spectrum of Fe2O3

with a laser irradiance of 9.8 � 1010 W/cm2 at the

Table 1. Summary of the operating parameters of theLI-O-TOFMS system

LaserWavelength 266, 532, 1064 nmPulse width 5 nsEnergy 0.5–10 mJRep. rate 5 HzSpot size 50 �m

Transmission systemNozzle 37.5 VFocus lens 1 �48 VFocus lens 2 �16 VFocus lens 3 �123 VSkimmer �24 VDCQ-up �5.9 VDCQ-down �3.7 VDCQ-left �6.1 VDCQ-right �2.4 VEinzel lens-sides 0 VEinzel lens-middle �18 V

Time-of-FlightRepelling pulse magnitude 750 VAcceleration potential �4848 VSteering plate potential �4460 V

Figure 2. Positive LI-TOFMS spectrum of Fe

2 3

wavelengths of (a) 1064, (b) 532, and (c) 266 nm.

wavelengths of 1064, 532, and 266 nm. In our study,laser wavelength did not have an obvious impact on theappearance of cluster ions. Comparing the results atlaser wavelengths of 532 nm (h� � 2.33eV) and 1064 nm(h� � 1.17 eV) (Figure 2a and b, respectively) with thatof 266 nm (Figure 2c), we could find that the detectedcluster ions and their distributions were nearly the same.With regard to the explanations, several mechanisms mayoccur simultaneously. First, thermal effect related to thesublimation of the irradiated solid surface was induced byhigh irradiance laser ablation. From area in the neighbor-ing of the laser impact, certain stable species (such as ironoxide clusters with the form of FemOm [33, 34] may beejected in the gas phase. At the same time, a large numberof electrons with significant kinetic energy were emittedmainly by thermal effect at high laser irradiance. As aresult, part of the cluster ions might be the productsfrom the ionization of neutral species by electrons,which are non-wavelength-dependent [10, 27]. Second,as the laser irradiation in our work was high andreached 9 � 1010 W/cm2, the central plasma tempera-ture was estimated to be 9 eV at 10 ns and 4.5 eV at100 ns after laser radiation [35–37]. Most of theneutral clusters were likely to be dissociated than beionized at the high temperature (e.g., bond energiesfor the loss of O and O2 from iron oxide clusters werebetween 3 and 5 eV [38], while ionization potentialswere 7 to 8 eV for the clusters [39]). As a result,particles at the central part of the expansion plasma

ith laser irradiance of 9.8 � 1010 W/cm2 and

O w

1230 YAN ET AL. J Am Soc Mass Spectrom 2010, 21, 1227–1234

may be fully atomization due to the high tempera-ture, which indicates that part of the neutral FeOcomes from the combination of iron and oxygenatoms, and also part of cluster ions are aggregationproducts after the plasma cools down. We think thatthe combination and aggregation could be the majorpathway for the cluster formation in high laser irra-diance. The original sample oxidation state can berevealed from the abundance ratios of the clusters,which could be iron oxide dependant. Thus, the ionformation mechanism in the subsequent discussion isof the primary concern for the differentiation.

Evaluation of Laser Irradiance

Although the laser irradiance has little effect on the typeof ions generated in the plasma, it influences ionintensity and distribution. With no buffer gas intro-duced, an increase in laser irradiance yielded a rise inthe ion intensity, of not only elemental iron, but oxideclusters, as higher laser irradiance would produce morethermal electrons which ionized the neutral species.Besides, signals of cluster ions with large metal por-tions, e.g., Fe3O3

� and Fe4O4�, increase noticeably over

other cluster ions with an increase in laser irradiance(solid lines in Figure 3). A probable reason for thisphenomenon is that higher laser irradiation could pro-mote the endothermic aggregation reaction that leads tothe production of large cluster ions with consumptionof precursor ions such as Fe� and FeO�. However, asthe laser irradiation increased up to 1010 W/cm2, thedistribution of ions remained unchanged. This may bedue to the limited number of ions that can be sampledby the nozzle. Hence, the laser irradiation used in ourspeciation analysis was less than 1011 W/cm2.

Figure 3. Integrated intensities of the ions as a f

in the study of Fe2O3.

Evaluation of Buffer Gas Pressure

The buffer gas pressure determines the mean free pathand collision frequency of ions and neutrals in theionization chamber. In previous work on GDMS [5], ahigher pressure yielded higher intensity peaks for ele-mental ions, while a lower pressure favored the sur-vival of larger clusters. Our studies in LIMS showedsimilar results (in Figure 4). The intensity of mostcluster ions decreased rapidly with an increase in buffergas pressure, while the signal of Fe� was slightlyenhanced. Frequent collisions between cluster ions andbuffer gas at high pressure are the cause of the phenom-enon described above, in which some cluster ions aredissociated in the collisions. Importantly, with thechange in buffer gas pressure, no obvious change wasfound in the distribution of cluster ions. The proportionof the dissociation of different cluster ions was nearlyidentical, which is significant because cluster ions, es-pecially their relative ratios, are important for specia-tion analysis.

The data above show the effects of the wavelength,irradiation, and buffer gas pressure on the signalintensity of elemental and cluster ions for Fe2O3. Theresults show that parametric adjustments in the ionsource will not change the variety of cluster ions andtheir relative ratios. Studies of parametric effectswere also carried out for Fe3O4, in which similartrends were found. Due to space constraints, the dataare not shown here.

Differentiation of Iron Oxides

The time-resolved spectra of Fe2O3 and Fe3O4 areshown in Figure 5. For both oxides, several FenOm

�

on of laser irradiance at the 1064 nm wavelength

uncti

in

1231J Am Soc Mass Spectrom 2010, 21, 1227–1234 DIRECT SPECIATION BY HIGH IRRADIANCE LIMS

cluster ions were detected with a similar amount anddistribution. No unique peaks could be found for eachoxidation state. For ion packets of different ions, dozensof �s are needed to transport the ion from the sourcethrough the interface and the electrostatic lenses, to therepelling region. The transport time increased with them/z because of the time-of-flight effect in the transmis-sion system. The duration of an ion packet of identicalions was even shorter, typically of only several �s.

The integrated mass spectrum of Fe2O3 and Fe3O4

are shown in Figure 6. The relative intensity of clusterions varies and is dependent on the particular ironoxide. Both of the spectra show strong iron peaks.Cluster ions such as FeO�, Fe2O0-2

�, Fe3O2,3�, and

Fe4O3,4� are also commonly found in our experiment,

which is different from previous results where clusterions containing up to two iron atoms were detected[40–42]. Two important processes could result in theabove phenomenon. One is that at high laser irradiancein our experiment, high-energy thermal electronswould ionize the large amount of species (such as FeO,Fe2O2, Fe3O3, and Fe4O4, etc.) emitted from the neigh-boring of the laser impact area. Besides, during theexpansion of the components, dissociation could occurbecause of the high collision rate and the residualinternal energy remaining in the ions, which may resultin the distribution of certain cluster ions, as indicated byMolek et al. [39] that FemOm

� gradually lose FeOneutral until Fe2O2

�, followed by Fe2O�, and ultimatelyto Fe�. The other reason for the increase in the varietyof cluster ions could be the effect of intense aggregationreactions in the plasma under high laser irradiation. Asillustrated by Maunit et al. [24, 25], laser irradiation ofiron oxides could generate singlet oxygen (1O2) in the

Figure 4. Integrated intensities of the ions as a fof 4.5 � 1010 W/cm2 at a wavelength of 1064 nm

plasma. 1O2 plays an important role in the aggregation

formation of oxide clusters. Under high laser irradiance,singlet oxygen may still be emitted; part of them is fromthe neighboring of the laser impacted area where thetemperature is high enough for thermal emission butstill inadequate for atomization and ionization. Thedimer Fe2

� was produced by a reaction between Fe� andneutral FeO, while the Fe2O

� ions arise from the reaction ofthe dimer Fe2

� with 1O2. However, in our experiment withhigh laser irradiance, in addition to the mechanism describedby Maunit et al., other aggregation pathways may alsoexist, such as Fe��O2¡FeO��O, Fe��FeO¡Fe2O�,FenOn

�� FeO¡FemOm� (n � 1,2,3, m � n � 1),

FenOn–1�� FeO¡FemOm–1

� (n � 2, 3, m � n � 1).Therefore, possibly part of the large cluster ions werefrom a complex aggregation reaction. Besides, as indi-cated by Molek [39], species in the form of (FeO)n

� havethe greatest stability of iron oxide clusters, which ex-plains why Fe3O3

� and Fe4O4� showed strong ion

peaks in our spectra for both iron oxides. It should benoted that O2

� peaks could be seen, while O� peakswere barely observed (Figure 6). This does not reflectthe amount of emitted oxygen due to the relatively lowionization energy (IE) of O2 (IE � 12.07 eV) comparedwith that of O (IE � 13.62 eV).

The differentiation of the two oxides was performedon the basis of abundance ratios of clusters. Specifically,the abundance ratios of two ion species in the sameseries (i.e., with the same number of iron atoms) areoften a criterion of choice for speciation [8, 21]. Theseratios are reported in Figure 7. For the ratio of Fe2

� toFe2O�, relatively higher abundances of the Fe2

� ion areindicative of Fe3O4, which is expected and consistentwith previously published results [24, 25]. Judging fromthe iron and oxygen content from the molecular for-

on of buffer gas pressure with a laser irradiationthe study of Fe2O3.

uncti

mula of Fe2O3 and Fe3O4, one could predict that the ion

2 3

/cm2

1232 YAN ET AL. J Am Soc Mass Spectrom 2010, 21, 1227–1234

ratio of Fe2� to Fe2O� in the two samples might be

close. However, the results in our research did notmatch this prediction as the ratio in the two sampleswas significantly different. This was not surprising, asseveral formation mechanisms of Fe2O� might exist:electron ionization of neutral species, dissociation fromlarge cluster ions, and combination and aggregationapproaches. Hence, one probable reason for the obviousdifference in the ratio of Fe2

� to Fe2O� was due to thevarying amounts of emitted oxygen during laser abla-tion of the two oxides and the fact that it is easier toproduce 1O2 from Fe2O3 during laser ablation thanFe3O4. According to past studies, the singlet oxygencame from the bulk structures of the samples. Thethermo-emission activity of 1O2 is inversely propor-tional to the specific surface area (SPA, which measuresthe total surface area per unit of mass) of investigatedsamples [43, 44], because large SPA inhibits the emis-sion of 1O2 from the volume localization. To verify thesinglet oxygen emission from the oxides, an experimentwas conducted to measure the SPA with a surface areaanalyzer (TriStar II 3020; Micromeritics, Norcross, GA,USA). The measured values were 4.69 and 8.21 cm2/g

Figure 5. Time resolved mass spectra of (a) Fionization with laser irradiance of 9.8 � 1010 W

for Fe2O3 and Fe3O4, respectively, which proves that a

larger amount of singlet oxygen was emitted fromFe2O3 than Fe3O4.

The same iron oxides were also analyzed by a laserdesorption ionization mass spectrometer (MicroFlexMALDI-TOFMS, with a laser wavelength of 337 nm,Bruker, Bremen, Germany). The sample plate was mod-ified such that the oxides, with no matrix added, couldbe held in the cavities on the plate. Averaged spectrawere acquired with the average of 100 single spectra,comparative to the accumulated spectra of 100 singlespectra in Figure 6 at high irradiance. Peaks can beobserved steadily in a narrow irradiance range of1.70–1.82 � 108 W/cm2. With the irradiance below 1.70 �108 W/cm2, the signals were too weak to be observed,while the background went high and the resolutionbecame extremely poor with the irradiance beyond 1.82 �108 W/cm2. The highest mass peak that appearedstably was Fe2O2

�. The ratios of Fe2�/Fe2O� and

Fe2O�/Fe2O2� are plotted in Figure 7 (dashed line

bar, with irradiance of 1.75 � 108 W/cm2). Their RSDsare about 20%.

Therefore, in low irradiance (�108 W/cm2) LIMS,spectrum is wavelength related [24, 25], the speciation

and (b) Fe3O4 as a function of time after laserat the 1064 nm wavelength.

e O

of iron oxides usually depends on the ratios of Fe2�/

.

1233J Am Soc Mass Spectrom 2010, 21, 1227–1234 DIRECT SPECIATION BY HIGH IRRADIANCE LIMS

Fe2O� and Fe2O�/Fe2O2�. At high irradiance (�109

W/cm2) LIMS, high-resolution spectra can be obtainedwith orthogonal geometry TOF analyzer; the ratios ofFe3O2

�/Fe3O3� and Fe4O3

�/Fe4O4� can be added (as

shown in Figure 7), which is independent of wave-length and further validates the speciation. Moreover,the cluster distribution under low irradiance reliesheavily on the neutral FeO; FeO is directly generatedthrough laser desorption, which is sensitive to the

Figure 6. Integrated spectra of (a) Fe2O3 and (9.8 � 1010 W/cm2 at the wavelength of 1064 nm

Figure 7. The ratios between the peak intensity of Fe2�/Fe2O�,

Fe2O�/Fe2O2�, Fe3O2

�/Fe3O3�, and Fe4O3

�/Fe4O4� for Fe2O3,

and Fe3O4. Solid line bars are for high irradiance of 9.8 � 1010

W/cm2 at the wavelength of 1064 nm using LI-O-TOFMS; dashline bars for low irradiance of 1.75 � 108 W/cm2 at the wavelength

of 337 nm using MALDI-TOFMS.

parametric change, such as the fluctuation of laser flux.In high irradiance LIMS, apart from electron ionizationof ablated neutral species, clusters were also formed byaggregation from the atomized iron oxides when theplasma cools down. As shown in Figure 7, the ratios arestable with an RSD of about 7%.

Conclusion

High irradiance laser ionization time-of-flight massspectrometry has been investigated in the direct specia-tion analysis of iron oxides. A vacuum laser ionizationsource can effectively reduce the dissociation processand keep the mass spectrum simple. Ion peaks can beidentified according to their m/z values and separatedby the delay time between the laser pulse and repellingpulse. With only iron, oxygen, and their clusters shownin the spectra, the samples can easily be identified asiron oxides. The clusters formed at high irradiancecould be the combination of several mechanisms: elec-tron ionization of neutral species, dissociation fromlarge cluster ions, and aggregation reactions. The ionabundance ratios of the clusters with the same numberof iron atoms can be used for the speciation of ironoxides, while, other than using the ratios of smallcluster ions, the ratios of Fe3O2

�/Fe3O3�, and Fe4O3

�/Fe4O4

�, can also be used for speciation with high laserirradiance. Unlike the laser desorption technique, stableratios of the ion species can be obtained under high

3O4 from LI-O-TOFMS with laser irradiance of

b) Fe

irradiance for the direct speciation of iron oxides, pos-

1234 YAN ET AL. J Am Soc Mass Spectrom 2010, 21, 1227–1234

sibly due to involvement of the atomization and aggre-gation mechanisms.

AcknowledgmentsThe authors gratefully acknowledge financial support from Na-tional 863 Program, the Natural Science Foundation of China, andFujian Province Department of Science and Technology.

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